1. Field of the Invention
The present invention relates to sheet stacking apparatuses that stack sheets, and sheet processing apparatuses and image forming apparatuses including the sheet stacking apparatuses.
2. Description of the Related Art
With the advancement in technology, image forming apparatuses form images on sheets at an increasing speed. Accordingly, sheet stacking apparatuses that stack sheets discharged from such image forming apparatuses are required to have a larger capacity and a higher productivity.
An example of such a sheet stacking apparatus is disclosed in US Unexamined Patent Application Publication No. 2007/0045948. In this apparatus, sheets discharged from a discharge port can be received alternately by a plurality of stacking trays that are movable vertically, whereby sheets can be stacked efficiently on the trays, realizing a large capacity of the apparatus as a whole.
However, the foregoing sheet stacking apparatus stacks not more than the maximum number of sheets stackable in the apparatus as a whole onto the vertically movable stacking trays. When the numbers of sheets stacked on all of the trays reach the maximum, the sheets need to be removed by a user. Therefore, during the removal of the sheets from all of the stacking trays, the entire operation of an image forming apparatus including the sheet stacking apparatus needs to be stopped, causing frequent occurrences of downtime and therefore reducing productivity.